As one of general methods for mass producing a glass preform for use in the fabrication of an optical fiber, the VAD (Vapor Phase Axial Deposition) method is known. The VAD method comprises depositing fine particles of glass generated in oxyhydrogen flame on a rotating starting member such as a glass plate or rod to form a cylindrical porous preform (soot preform) and sintering said porous preform to obtain a transparent glass preform for use in the fabrication of an optical fiber.
In the VAD method, for sintering the porous preform to convert it into transparent glass, the preform should be heated in an atmosphere of an inert gas (e.g. helium and argon) to a temperature of 1,600.degree. C. or higher. As a heating furnace for sintering the preform, usually a heating furnace having a carbon heater is used. What should be taken care of when sintering the preform in such heating furnace is inclusion of transition metals such as copper or iron and water. When 1 (one) ppb or larger of the transition metal is included in the glass preform, transmission loss wavelength characteristics of the fabricated optical fiber is greatly deteriorated in an entire wavelength range. When 0.1 ppm or larger of water is included in the preform, the characteristics of the fabricated optical fiber is impaired in a longer wavelength range.
Therefore, the porous preform is usually dehydrated before or during vitrification. As a dehydration method, it is known to heat the porous preform at a high temperature in an atmosphere of an inert gas containing a chlorine-containing gas, a fluorine-containing gas, etc. When the fluorine-containing gas is used, not only the porous preform is dehydrated but also fluorine is added to the porous preform. When the fluorine is added to the porous preform, a refractive index profile which is essential to the optical fiber is advantageously adjusted. In this connection, Japanese Patent Publication No. 15682/1980 and Japanese Patent Kokai Publication No. 67533/1980 can be referred. These publications will be discussed below.
The treatment with the fluorine-containing gas is carried out in the heating furnace before or simultaneously with vitrification. To prevent wastage of the carbon heater due to moisture or oxygen which is generated during heating of the preform, a muffle tube is installed for separating the carbon heater and the sintering atmosphere. As the muffle tube, an alumina made one is conventionally used (cf. Japanese Patent Publication No. 40096/1982 and U.S. Pat. No. 4,338,111). However, when the alumina made muffle tube is used, alkali components contained in alumina float in the heating atmosphere at high temperature and adhere to a surface of the porous preform to form a cristobalite layer.
Then, a quartz made muffle tube has been practically used. In comparison with the alumina made muffle tube, the use of the quartz made muffle tube gives following advantages:
1. The quartz has better mechanical processing accuracy and therefore airtightness of the atmosphere is maintained so that the soot preform is effectively dehydrated.
2. The quartz made muffle tube contains few impurities such as iron and alkali and is much purer than the alumina made muffle tube.
3. The glass preform produced by means of the quartz made muffle tube does not suffer from surface devitrification caused by alkali.
4. The quartz made muffle tube hardly suffers from. thermal breakage (breakage due to thermal shock).
5. When the fluorine-containing gas is used, no contaminating gas such as AlF.sub.3 and the like is generated. Although gaseous SiF.sub.4 is generated, it does not act as an impurity which has adverse influence on the glass preform.
The methods utilizing the quartz made muffle tube are described in detail in Japanese Patent Publication Nos. 58299/1983 and 42136/1983 and Japanese Patent Kokai Publication No. 86049/1985.
If copper and iron are contained in the quartz glass, they easily react with the chlorine-containing gas contained in the dehydration atmosphere according to the following reaction formulae to form volatile chlorides, which penetrate into the porous preform and severely deteriorate the transmission loss characteristics of the finally fabricated optical fiber. This is a new problem associated with the quartz made muffle tube. ##STR1##
Another problem is that, since copper tends to easily diffuse in the quartz glass at a high temperature, copper which is liberated from the heating furnace itself or the heater penetrates through the muffle tube and contaminates the glass preform.
Further, the fluorine-containing gas is decomposed or reacts to form F.sub.2 gas or HF gas. These gases react with the quartz glass according to the following reaction formulae to generate SiF.sub.4 gas, and by these reactions, the quartz glass is etched: EQU SiO.sub.2 +2F.sub.2 .fwdarw.SiF.sub.4 +O.sub.2 EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +2H.sub.2 O
Because of such etching, copper and iron present inside the quartz glass appear on the surface and contaminate the porous preform. In addition, by etching, pin holes are formed in the quartz made muffle tube, which is a cause of intake of environmental air or leakage of the dehydration atmosphere. These are not advantageous for the production method.
Furthermore, the quartz glass tube has a very bad problem that it tends to easily deform at a high temperature. That is, when the quartz glass is kept at about 1,300.degree. C. for a long time, it deforms due to viscous flow. In addition, when it is used at a temperature of 1,150.degree. C. or higher for a long time, it is devitrified, and once the furnace temperature is lowered, strain is generated due to difference of thermal expansion coefficient between the glass phase and the devitrified phase and finally breaks the tube.
Meanwhile, the glass preform for the optical fiber comprises a core part and a cladding part, and the core part occupies a center portion of the glass preform and has a higher refractive index than the cladding part so as to transmit light. In refractive index structures of a single mode optical fiber and a multi-mode fiber shown in FIG. 1A and 1B, respectively, "A" part and "B" part correspond to the core part and the cladding part, respectively.
To form refractive index difference between the core and the cladding, the refractive index of the core is increased and/or that of the cladding is decreased.
"Refractive index difference" herein used is intended to mean a difference of refractive index between a certain glass and a pure silica.
To increase the refractive index of the core part, a refractive index increasing dopant such as GeO.sub.2, Al.sub.2 O.sub.3 and TiO.sub.2 is added to a glass forming raw material during synthesis of the quartz glass so that an element such as Ge, Al and Ti is added to the glass. However, if such metal oxide is used, following defects will arise:
In proportional to the increase of the amount of the added dopant, light scattering (Rayleigh scattering) due to the dopant, which is not preferable for light scattering, increases. If a large amount of the dopant is added, bubbles or crystalline phases are generated in the glass preform. For example, when GeO.sub.2 is used, bubbles of GeO gas tends to form, and when Al.sub.2 O.sub.3 is used, clusters of Al.sub.2 O.sub.3 tends to form. Such bubbles or crystalline phases have undesired influence on light transmission characteristics and also strength of the optical fiber.
Therefore, it is understood that a composition of the core part preferably consists of the pure quartz glass or quartz base glass containing the dopant in an as small as possible amount.
As one of measures for achieving the refractive index difference between the core part and the cladding part with overcoming the above various drawbacks associated with the addition of the dopant to the core part, it is proposed to provide a glass preform for an optical fiber comprising a cladding part to which fluorine, which decreases the refractive index, is added. One of the advantages achieved by the use of fluorine as the dopant is that the core part can be made of the pure quartz or quartz base glass containing an as small as possible amount of the dopant since the refractive index of the cladding can be made lower than that of the pure quartz. FIGS. 2A-2D show a refractive index structure of the quartz base glass optical fiber comprising a cladding to which fluorine is added. By such structure, light scattering (Rayleigh scattering) due to the dopant contained in the core through which light propagates is reduced and the core has preferable properties as a light transmitting guide.
Further, a resource for fluorine is richer than that for other dopants such as GeO.sub.2, and purification of a raw material is easy, which is economically advantageous. In addition, the fluorine gas not only acts as the dopant for adjusting the refractive index of the glass but also acts as an excellent dehydrant for removing moisture contained in the soot preform. This is also one of the characteristics of fluorine.
For adding (or doping) fluorine to the quartz glass, several methods have been proposed.
Firstly, Japanese Patent Publication No. 15682/1980 describes a method comprising supplying the fluorine-containing gas in a gaseous phase synthesis of glass so as to add fluorine to the glass. Although this method can add fluorine to the glass, it has such drawbacks that a glass deposition efficiency and fluorine addition efficiency (doping yield) are both low. The reason for this may be that in the flame hydrolysis which utilizes oxyhydrogen flame, moisture in the flame and the fluorine-containing gas such as SF.sub.6 react according the reaction formula (1) to generate hydrogen fluoride (HF) gas: EQU SF.sub.6 +3H.sub.2 O.fwdarw.SO.sub.3 +6HF (1)
Since the generated HF gas is stable, almost all the fluorine-containing gas is converted to the HF gas at high temperature as long as the moisture is present, and only a slight amount of the remaining fluorine-containing gas is used as the dopant.
The HF gas etches the glass, particularly quartz and reacts with the fine particles of the glass synthesized in the flame according to the following reaction formulae (2) and (3): EQU SiO.sub.2 (s)+2HF(g).fwdarw.SiOF.sub.2 (g)+H.sub.2 O(g) (2) EQU Sio.sub.2 (s)+4HF(g)+2H.sub.2 O(g) (3)
wherein (s) and (g) stand for a gas and a solid, respectively. Thereby, the synthesized fine particles of the glass are consumed so that the deposition efficiency is decreased.
Accordingly, increase of addition of the fluorine-containing gas results in decrease of the deposition rate of the soot particles.
Secondly, Japanese Patent Kokai Publication No. 67533/1980 discloses a method comprising synthesizing fine particles of the glass by flame hydrolysis, depositing them to form a soot preform, heating the formed soot preform in an atmosphere comprising a fluorine-containing gas to dope fluorine to the soot whereby a glass preform containing the fluorine is produced.
However, this method also has several drawbacks. In one embodiment of the method described in said Japanese Patent Kokai Publication, the soot preform is heated in the atmosphere comprising the fluorine-containing gas at a temperature of not higher that 1,000.degree. C. However, the addition rate of the fluorine is low and sometimes copper and iron are present in the finally fabricated optical fiber. Copper and iron are know to cause absorption loss which is a cause of increase of transmission loss.
It is also described to treat the soot preform in the gaseous atmosphere comprising the fluorine-containing gas at a temperature of not lower than 1,400.degree. C. However, a surface of the produced glass preform is etched, and also the muffle tube such as the quartz made muffle tube for maintaining the atmosphere is sometimes severely damaged by etching. Such etching of the muffle tube is one of the causes for increasing contamination of the soot preform with the impurities in the muffle tube.
In addition, the fabricated optical fiber in said Japanese Patent Kokai Publication suffers from change of absorption loss with time due to hydroxyl groups, and the absorption loss greatly increases at high temperature.
To overcome such problems, Japanese Patent Kokai Publication No. 239337/1985 discloses a method in which SiF.sub.4 is used as the fluorine-containing gas.
SiF.sub.4 is only one fluorine-containing gas which does not etch the soot preform and the quartz glass made muffle tube so that it does not induce the breakage of the quartz glass made muffle tube due to etching.
However, in addition to the above described drawbacks, the quartz glass made muffle tube has following drawbacks. Through the quartz, impurities such as alkali and copper penetrate. If a slight amount of water is present, it reacts with SiF.sub.4 to form HF which etches the quartz glass made muffle tube so that the impurities contained in the muffle tube material may contaminate the soot preform. Penetration of the impurities can be prevented by lining the whole muffle tube with a highly pure material. But, the lining increases the production cost of the muffle tube and is uneconomical. To prevent the etching of the muffle tube, the soot preform and the muffle tube are thoroughly dried to remove the moisture before the supply of SiF.sub.4 in the muffle tube, which requires an airtight equipment or careful operation.
As a material which hardly reacts with the fluorine-containing gas or the chlorine-containing gas, carbon is contemplated. The carbon does not either react with SF.sub.6, C.sub.2 F.sub.6, CF.sub.4 and the like which easily react with the quartz. Of course, the carbon does not react with SiF.sub.4.
Japanese Patent Publication No. 28852/1981 suggests the use of a carbon made muffle tube in an atmosphere comprising the fluorine-containing gas such as F.sub.2, although no working example is described.
However, the carbon has following drawbacks:
1. Since the carbon has minute pores, gases can penetrate therethrough. Permeability of nitrogen through the carbon is 10.sup.6 times larger than through the quarts glass.
2. The carbon is easily oxidized and, at a temperature not lower than 400.degree. C., it easily reacts with oxygen to form CO.sub.2 or CO.
To prevent oxidation, it has been proposed to form a layer of ceramics such as SiC, Al.sub.2 O.sub.3 and BN on an inner wall of the carbon muffle tube. Although the ceramics layer prevents the oxidation, it disadvantageously reacts with at least one of the chlorine-containing gas and the fluorine-containing gas. Impurities generated by such reaction devitrify the soot preform and generate bubbles in the soot preform.
Although F.sub.2 gas has no possibility to liberate carbon or sulfur, it explosively reacts with water. Therefore, the F.sub.2 gas is not suitable as a fluorine-doping gas.
Since the carbon is a material having large gas permeability as described above, the gas goes in and out through the wall of the muffle tube so that the moisture in the air penetrates into the muffle tube through the wall. Therefore, the glass preform contains a comparatively large amount of water and in turn the hydroxyl groups. In addition, the gasses such as Cl.sub.2 and SiF.sub.4 are released outside the furnace and may pollute a work environment, and impurities (e.g. copper and iron) may penetrate into the furnace from the outside. These defects can be considerably overcome by increasing the thickness of carbon, but still not completely.
As explained above, the addition of fluorine to the quartz glass of the cladding part by the conventional methods encounters various difficulties.
In view of such circumstances, the present invention intends to solve the problems of the conventional muffle tube which is used in dehydration of the preform for the optical fiber and addition of the fluorine to the preform and to provide a muffle tube for producing the glass preform for the optical fiber, which has improved durability and long life and can prevent the penetration of the air into the muffle tube.